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In the old days, why didn't aircraft use simpler systems to land on a Carrier?
In the old days (during and before WW2), Carriers used arresting wires to help aircraft land. In fact this is still used today for jet-based carriers, but I want to ask about the old days with piston-prop aircraft because they go slower and generally make this easier.
I'm pretty sure even back in WW2, most carrier aircraft had tailhooks that grabbed the wire. This is so you "brake in the back", not the front. If you braked in the front, you are much more likely to nose forward or spin around.
Well, I've seen plenty of footage where that happens because something in the front of the aircraft, the nose or front wheels, gets tangled up in those wires, causing bad accidents. Here's a good compilation that has all sorts of landing accidents, including with the wire. What tends to happen is the wire is snagged wrong and the aircraft nose dives into the deck, wrecking the propeller, and/or the aircraft goes spinning around.
I can think of several alternatives to an arresting wire:

Reverse Pitch Propellers (reverse thrust)
Powerful Brakes in the rear wheel, or all wheels
Drag Chute
Air Brake

To me the simplest solution seems to be the last one, an Air Brake. I've seen this on modern jets like the Su-27 (@ 5:06). It's basically a big flat panel that swings out and creates enormous drag. In the Su-27 they keep it near the center of mass so it doesn't induce large torques, but you can put it in the rear half of the aircraft too. That will "brake in the back" and I think it won't require any more airframe strengthening than a tailhook, since that brakes in the back too.
So why not use this simpler solution? For reference, an Essex Class Carrier has a runway of about 260 m. I actually could not find normal "landing distance" that a small aircraft (bf 109, spitfire, etc.) needs once it touches down. If someone can find that, we would be closer to an answer.

So you want to add some complicated, heavy system to a fighter aircraft rather than some simple system on the boat? Loosing a few props seems like a bargain against loosing aircraft when a few pounds or aerodynamics means the difference between coming home and a chaplain at your families door... Early fighters weren't very complicated, the Spitfire was made out of plywood.

The airbrake only creates enormous drag at enormous speed. When the aircraft slows down, the brake becomes less and less effective. The arrestor wire, on the other hand, works at all speeds.

Funny that you think adding a chute is simple at that time. Too many things we take for granted nowadays.

Never mind how a reversable pitch propeller is supposedly simple, but a wire and hook is supposedly complex...

@MichaelKjörling: Probably because variable-pitch propellers are standard on all propeller-driven aircraft, and allowing the propeller to be given a negative pitch is a natural extension of that.

"something in the front of the aircraft, the nose or front wheels, gets tangled up in those wires" - Today, the aircrafts approach the carrier with a specific Angle-of-Attack (AoA) which assures that the tailwheel touches down and the hook grabs a wire before the nose comes down.

@jklingler Accidents happen. You can touchdown a bit early, risking nose gear entanglement. You can specify every little detail about a correct approach, but accidents can still happen.

@DrZ214 There are multiple systems to prevent that from happening. Ofc human error is a factor but with the LSO's that guide and talk the planes down, visual cues in the HUD and the "Ball" next to the carrier box, dangerous situations can be identified as such before accidents happen and according actions can be taken. IMHO, thanks to all those measures, one cannot compare the situation from the old days to today.

@RonBeyer What is so complicated and heavy about an air brake? It's just an extra control surface. Besides, some fighters (Su-27) already have one, so it's not like it's unnacceptable weight for an air-superiority fighter... loosing aircraft when a few pounds or aerodynamics means the difference between coming home and a chaplain... Completely disagree. If this were strictly true, then the USAF would have been desperately trying to copy the Japanese Zero, or even improve it by making it lighter. As it happened, the Hellcat beat the Zero later in the war, and it was significantly heavier.

@RonBeyer ...The whole aircraft superiority argument can be taken too far. There are many factors besides aircraft performance. Pilot training is a big one. In fact the Zero vs Wildcat then Hellcat example I just cited is a great example of pilot skills. The high skill of Japanese Pilots was not replaced quickly enough after terrible losses at Midway, while the Americans got better and better. So I'm sorry but I have to almost completely disagree with your point. Marginal performance differences are not big enough of a factor. The best aircraft of any era are usually similar enough anyway.

@jklingler one cannot compare the situation from the old days to today No argument there. I was asking about WW2 and pre-WW2 anyway.

@Sean: Why on Earth do you think variable pitch propellors are "standard on all propeller-driven aircraft"?  Many planes,  probably including almost all the ones flown by those of us who aren't professional pilots, have fixed-pitch props.

@jamesqf https://aviation.stackexchange.com/questions/53126/did-ww2-era-aircraft-have-constant-speed-or-variable-pitch-props They were the norm on warplanes of WW2. Also, where did you get the quote "standard on all propeller-driven aircraft"? I never said that phrase. Can't find anyone else here who said it either.

@DrZ214: I found that phrase (and copy-pasted it into my comment) in the comment by Sean.  It may be the case that most or all WWII warplanes had variable pitch props, but WWII warplanes are a small subset of ALL planes.

@RonBeyer: That was the Mosquito - Spitfires were made of aluminium. They were even the first carrier planes that used aluminium extensively and the first batch corroded away in the salty air within 6 months. That led to the development of the seawater-resistant alloys of the 5000 series.

"Many planes, probably including almost all the ones flown by those of us who aren't professional pilots, have fixed-pitch props. "  Disagree - I would opine that most airplanes with 180HP and greater engines have a variable pitch/constant speed prop. And there's quite a few of those out there, flown by amateur pilots in 4-seat and 6-seat aircraft.

Proper pilot technique is critical no matter what system you use.  Most of the mishaps in that video would not have been avoided by using one of the alternate methods you suggest.

The Essex class carriers might have had 260 m overall length, but only a little less than half was actually available for the landing. Most of the accidents where the gear gets tangled in something look like the hook simply misses the wires and then the gear gets tangled in the barrier midway the deck to prevent the landing plane from crashing into the planes preparing for launch on the forward deck.

The benefit of a cable is that it's a mechanical means of arresting the aircraft. You either catch the cable and stop, or miss and go around. Aside from breakage and malfunction, there is no in-between. If braking is left entirely up to the pilot, it's up to them to engage all the right functions quickly enough, and in sufficient amounts. If they misjudge, they end up going into the water.
Cables are also effective at stopping aircraft quickly. This means less deck space is needed for landing, and with the introduction of the angled landing area, allows aircraft to be launched and recovered at the same time. Only a tail hook is needed to be added to the aircraft, which adds great braking effectiveness for relatively little weight. Other options have reasons they would be less effective than cables, with greater penalties in areas such as weight.
Reverse Pitch Propellers (reverse thrust)
This can be an effective way to help slow a propeller aircraft down. The downside is that it's up to the pilot to engage the reverse pitch and in a sufficient amount. Single engine aircraft can be hard to control especially if higher power settings are needed while in reverse.
Powerful Brakes in the rear wheel, or all wheels
Brakes are limited by the friction between the wheel and the ground. On carriers, this was generally wood, later replaced by metal. Especially when wet, this does not provide very much friction. Increasing downforce (see the air brake section) can help, but only to an extent. And as you noted, especially with tail draggers used on early carriers, aircraft would nose over if they braked too hard. The tail wheel doesn't have much weight on it, so would not be effective in braking.
Drag Chute
You'll notice that pretty much the only aircraft that use drag chutes are aircraft that land at high speeds and already need long runways. This is where you get the most benefit from a chute; long landing rollouts at high speed. As speed is reduced, they become less effective. You need to spend the time recovering, repairing/replacing, and re-packing the chute after each landing. They are also going to be less reliable than a tail hook. If it fails, you can't land on the carrier, you may just have to bail out.
Air Brake
Air brakes are similar to drag chutes in that they work best at high speed. While they are are not a consumable item, the drawback is that they have much less area, and therefore much less effectiveness than chutes. They work best when they can also provide downforce to improve braking friction, but still need high speeds and long distance to really provide benefit.
Carrier aviation in World War II was certainly dangerous. Many pilots died in accidents rather than in combat. But the military was still learning about how useful carriers would be in combat, and figuring out what the best configuration for carriers and their aircraft would be. Carrier aviation has come a long way since then, and cables are still the go-to system for recovering aircraft that don't at least have some kind of STOVL capability.

On note: going around after missing a wire was not an option for the WWII carrier operations the question is asking about. It was catch a wire or have a barrier crash. See https://en.wikipedia.org/wiki/Bolter_(aeronautics)

Aerodynamic braking is of no use at low speeds: the braking force is proportional to airspeed squared, so at low airspeed you get a very low braking force. And that is not what is required in a carrier: the minimum airspeed is the stall speed, and this needs to be reduced to ZERO ground speed in a hurry, when on the deck.
This answer explains why the P51 Mustang could not be used for aircraft carriers: the stall speed was too high, even for the arresting hook and wire setup. Deploying an air brake either stalls the aircraft before landing or overshoots the deck in a hurry after touchdown.

In addition to what the others have already said, there was no need. There is a balance between the cost of something better and the actual benefits of it, primarily in terms of maintenance required, weight penalty to the aircraft, risk reduction, the amount of stopping distance required and sorties per hour.
While not perfect, the wire allowed for reasonably short stopping distances without adding too much equipment to the aircraft. During WW2 fighters required good performance to have a chance at shooting down their opponents. Adding extra and more complicated gear might reduce their performance while adding little of value if the only result is to make landings a bit easier. If the equipment is too complex, it requires more maintenance and might be easier to damage during combat.
Aerodynamic braking or reverse thrust requires a longer stopping distance, resulting in having to build larger carriers. This then drives the cost, making them a bigger and more critical to defend/keep out of harm's way as you can't build that many. And even today using STOVL, the aircraft is penalized with a lower payload and shorter range compared to conventional aircraft.

To add to the disadvantage of a drag chute. The now retired USCG HU-25 Guardian (military version of Dassault Falcon 20) had a drag chute attached to the tail cone for emergency landings or short runways. This was because the engines the coast guard used did not have reverse thrust capability.
This drag chute was only effective to about 60 knots. It was designed to quickly (and it did, you felt some dramatic deceleration) from initial touchdown speed to 70-60 knots. At that point it did very little, but the wheel  brakes took over at this speed anyways. The HU-25 also had wing mounted airbrakes/spoilers. Again, these only assisted with slowing down. They were more effective at high speeds in flight to remove lift from the wing.
Even together, both of these systems still did not allow a landing anywhere near short enough for a carrier, unless you wanted to replace brakes and wheel's after every landings (they WILL catch fire, I've seen it)
As speed decreases so does effective of drag. So anything relying on a drag component will be ineffective when you really need it.

The WWII fighters weren't landing too much faster than 60 knots, so they'd see the not-effective speeds for such a chute for nearly all of their time on the carrier deck.  Thanks for adding specific speeds to the general discussion of chutes & airbrakes above -- good info.  And, welcome to Av.SE!

Several issues favored the use of arresting gear over good STOL capable aircraft, chiefly, adding STOL characteristics is often at odds with the requirements for a good fighter ie a low drag, highly maneuverable airframe couples with a high power to weight ratio engine.  Adding features like a reversible propeller added excess weight to the design as well.  Arresting gear also offers the option to recover aircraft at heavier weights or returning with unexpended ordnance, as opposed to having to jettison fuel and stores prior to landing.
There there’s the issue of straight deck carrier operations, namely that they often made use of only half of the flight deck for landing operations while the mid and fed sections of the flight deck was used for refueling, rearming and launching of aircraft.  Landings on straight deck ships were a one way in affair; once the LSO gave you the cut signal, you were committed.  Therefore the ship made use of multiple arresting gear plus a barricade to ensure the aircraft would stop before it plowed into personnel or airplanes on the midship.  Going into the postwar period, jet aircraft increased in size and power requiring more space to stop them in which led to the development of an angled landing area.  This offered both the real estate to arrest these heavier faster aircraft and also facilitated the option of a go-around in the event of a bad approach or a bolter.


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In WW2, were self-sealing fuel tanks really cost-beneficial?
I just read this question and answer, which seems to confirm my fears that an incendiary bullet will still destroy you if it hits your fuel. In other words, a self-sealing fuel tank will not protect against incendiary bullets. The wing will be punctured, and the fuel will either explode, or catch fire and remain on fire. (Correct me if I'm wrong.)
So the question is, why have them at all? It seems so easy to defeat the system by just using incendiary bullets, which must be a whole lot cheaper than the rubbery fuel tank.
I know there is accident protection and flak fragmentation protection. However, if you're taking flak, it seems to me you've got many problem-areas under the aircraft to worry about. I remember reading about a lot of bomber pilots in WW2, who took hits in the legs from flak, and some of them started sitting on their flak jackets all the time.
I realize this is a complex question, because cost-benefit analysis is a complex thing. But maybe someone can explain the rationale held at the time. Self-sealing fuel tanks added cost and weight, yet seem easily defeatable with incendiary bullets. Maybe I am missing something?

Your description of a self-sealing fuel tank remaining on fire when lit is the exact situation of the sealing mechanism failing. As soon as the tank is punctured, regardless of a fire or not, the rubber in the middle layer will react with the fuel and swell until the hole is reasonably obstructed. This both extinguishes the fire and seals the leak. Consider a case study: in the Pacific theater, Japanese fighter designs did without self sealing tanks for the cost and weight saving benefits. American naval designs such as the Hellcat however, had every possible damage-control system of the day installed everywhere possible to make the planes as rugged as they could. In the end, Japan mainly lost control of their own airspace due to the fact that they simply ran out of materials to build new planes from. The rate at which they lost planes was incredibly high due to both the small .50 caliber rounds' ability to set the unprotected tanks on fire, and the poor training of Japanese pilots towards the end of the war. If you look at statistics, per battle the IJAAF and the IJN lost more planes per squadron and battle group than any other country's air service in the war. In fact, many of the IJAAF's main fighters including the Mistubishi A6M Zero and the Kawasaki Ki-61 Hien were infamous among pilots of both sides of their ability to catch on fire from what should have been inconsequential hits and particularly their inability to stop burning. Another point of interest is the design of the self sealing fuel tank:

This all goes to prove strongly that NO, self sealing fuel tanks are a definite must in combat airplanes, and a very important safety feature in other aircraft types. I'm confused however as to why your question switches from bullets (incendiary?) to flak as anti-aircraft guns such as the Flakvierling88 were not meant to ignite fuel tanks, they were supposed to destroy the whole plane, fuel tanks and all. If you want some more numerical data backing up my position, Wikipedia references many sites that give a bit more data on why self sealing tanks are a must for fighter planes. The Defense Systems Information Analysis Center also provides some interesting information on the design and how they weren't all too much of a cost/weight penalty later in the war. Hope this answers most of what you're asking.
In addition, there is a specific part of the article of my second link I would like to draw attention to:

This simple composition and design, which are inexpensive, lightweight, and easily produced in large volumes, confronts the deceptively complex ballistic dynamics of a bullet perforating a fuel tank.  When a normally oriented bullet pierces a fuel tank, it leaves a small residual hole less than the diameter of the projectile.  But this small hole is just the beginning of the challenge.  The bullet, travelling at 3,000 ft/s, begins to unload its tremendous kinetic energy on the fuel inside.  This unloading generates a high-velocity wave of pressure in the fuel that is reflected back on the wall milliseconds after the bullet tears through.  This pressure wave, known as hydrodynamic ram, blasts a jet of fuel back through the entrance wound.  The bullet, still moving at a high velocity, and now followed by the same pressure wave, begins to tumble as it travels through the fuel.  When it reaches the back wall of the tank, the bullet erupts sideways through the material and leaves an elongated gash that is often torn further by the trailing wake of hydrodynamic ram.
Initial fuel tank prototypes tested by Navy engineers were discouraging failures.  Engineers had not anticipated the challenging dynamics, and often the entire back walls of prototype tanks were blown out by the hydrodynamic pressure wave.  However, these early development tests shed some light on the mechanisms that brought down so many pilots and aircraft during the World War I.  The Navy and rubber company engineers persisted and established remarkably effective design features and manufacturing processes that ultimately saved untold lives not just in World War II but in each of the armed conflicts since then.

This statement strongly backs up the cost/weight penalty not being an issue position, if this clears anything up.

I brought up flak because the shrapnel could pierce a wing with a small hole, so self-sealing would obviously fix that. I'm still not 100% clear on incendiary bullets, tho. An inc. bullet that pierces into the tank is going to ignite something, regardless of what kind of tank it is. I noticed the wiki article said ...were able to withstand .50 in (12.7 mm) bullets and, on occasion, 20 mm (0.79 in) cannon shells. but it didn't say if those were inc. bullets. I guess I should have specified something about size. Obviously there is a limit to how big a hole a self-sealing tank could seal.

Keep in mind that you need three things to start a fire: an ignition source, fuel, and oxygen. An incendiary bullet provides the ignition source in the form of heat, and the aircraft provides the fuel, but you won't get a fire unless there is also adequate oxygen in the tank. Self-sealing tanks will reduce or eliminate external oxygen sources by sealing the entry holes, so you only have to worry if there is oxygen already present in the tank/bladder. Even if there is, it will be consumed relatively quickly and the fire will burn itself out -possibly before further damage is incurred.

Yes but keep in mind, 20mm shells had a purpose more akin to flak than a smaller mg caliber round. German shells especially were designed to rip apart the structure i.e. the minengeschoss 20mm HE round.

Actually, Japan had a fair number of aircraft at war's end. It was pilots that they ran out of - takes time to train one. During the Battle of Leyte Gulf, their four remaining carriers were used as decoys, because they didn't have enough trained carrier pilots to outfit the carriers with an attack group.

An incendiary bullet penetrating a self sealing tank won't necessarily cause a fire. Gasoline needs oxygen to burn. The Allies developed a self sealing bladder that not only plugged holes, but gradually collapsed as the fuel was used up, to suppress the mix of air and gasoline vapor that replaces the liquid fuel as the fuel is consumed and the level drops... in an unsealed tank.
Why is that important? A gasoline vapor/air mix in an enclosed space doesn't burn... it explodes with great force. In the case of a partially empty aircraft fuel tank, enough force to blow the plane apart.
This is why the Japanese aircraft in WW2, that didn't use self sealing tanks, often exploded when hit. A half empty unsealed tank could blow up from a single incendiary hit, if the vapor/oxygen mix was conducive to an explosion (not always, but often). Allied aircraft, that took measures to suppress the creation of gas/air vapor in an enclosed fuel tank, weren't nearly as prone to blowing up with only a few bullet hits.
Even if a vapor explosion didn't happen, and it didn't always happen, the fuel spraying out from an unsealed tank that had been hit could be ignited by anything... the next bullet, electric sparks from damaged wiring, even the flames coming out of the aircraft's engine exhaust... doesn't take a lot to ignite highly volatile gasoline vapor.
Finally, consider what Japan did with its later aircraft designs.
Towards the end of the war, Japan brought out some new fighters. The most capable, and an equal of the better US fighters, was the Kawanishi N1K2 Shiden Kai, which featured 4 20mm cannon, armor plate and bulletproof glass for the pilot, and... self sealing fuel tanks.
Late in the war, it appears that even Japan, the one nation who did not use self sealing fuel tanks on their aircraft, changed their mind.
Additional notes: two somewhat recent air disasters illustrate how the problems of WW2 combat aircraft are still with us.
The hydro shock problem of a bullet hitting a full fuel tank and creating a shock wave was revisited in the Air France Concorde crash. A piece of a tire flew up and hit the Concorde's fuel tank. It didn't penetrate the tank, but the shock wave caused the tank to break open and leak a large amount of fuel, that caught fire from the Concorde's afterburners.
After this crash, the Concordes were given new fuel tanks with a bladder similar to the self sealing bladders of WW2, to keep the tank sealed in the event of hydro shock from an impact, and prevent this from happening again.
TWA flight 800, a 747 that crashed off of Long Island, is believed to have been brought down by an explosion of its center fuel tank. The center tank was nearly empty, and had an air conditioning unit next to it that had been running for some time while the plane was on the ground, heating up the fuel vapor/air mixture. While this was kerosene and not the more volatile gasoline, the nearly empty tank exploded not long after the airplane departed the airport, possibly due to an electrical fault plus the high temperature of the vapor mix.
After this crash, commercial airliners were required to vent inert gas into the tanks rather than atmosphere. Venting with inert gas is also required procedure for oil tankers, after a few of them exploded when a spark ignited the oil vapor in nearly empty oil tanks.

Misread that as "420mm cannon" and was mightily impressed for a moment. :-)

P.S., do you have a source for this: Japan, the one nation who did not use self sealing fuel tanks on their aircraft, changed their mind. I've never heard of Soviet aircraft from WW2 using them, but then, I've hardly read much about the R&D of Soviet WW2 aircraft. If they did use self-sealing fuel tanks, I want to read about it.

@DrZ214 Self-sealing tanks don't all shrink.  The self-sealing functionality is orthogonal to bladderized tanks, though it's easier to make a bladderized tank self-sealing since it must be rubber anyway.

@DrZ214 The self-sealing part is important, even without the tank being bladderized. If the tank is self-sealing, even if there is air in the tank, any ignited fuel will quickly exhaust the available oxygen, which will extinguish the fire due to lack of oxygen. The exception, of course, is if the fuel vapor/air mixture actually explodes... preventing that is where the bladderized tank comes into play.

Based on DoD Ballistic Research Laboratories document Effectiveness of Incendiary Ammunition Against Aircraft Fuel Tanks, especially page 10:

It should be noted that both the fracture of the nose and the tumbling
  of incendiary projectiles influence the self-sealing property of tanks.
However, so-called "plugging" of the tank, where a piece of the tank is actually removed, will usually result in its defeat. An incendiary bullet may assist this "plugging" action by burning of rubber and by virtue of the break-up of the nose, resulting in a penetration by a jagged blunt nose.

So it seems that a hit by incendiary bullet is worse than a hit by a normal bullet, but there is still possibility that the tank will be able to self-extinguish. After all there will not be that much oxygen inside the tank, so the fire will eventually run out of it if the self-sealing works. There is also the option of filling the tanks with nitrogen or other non-combustible gas, apparently used at least on Russian fighters.
On page 61, on fuel tank fires, it is noted:

If the opening caused by the round is small, a low-order explosion may occur by the expansion of the resulting gases. The fire may extinguish itself due to lack of oxygen and no serious damage to the structure will result.
If the opening is large enough, air can enter to support combustion and a fire may result which may cause a kill.

Inerted Fuel Tanks (i.e. filling them with nitrogren/other) have I think been standard in military applications for years (everyone, not just the Russians), and more recently have been mandated in commercial applications. I was surprised to read that they only keep the oxygen content down to 10% or so, but I suppose that implies that fires don't readily start without an abundance, suggesting igniting the tanks is harder than the OP might be presuming. (Inert filler also has the benefit of keeping the fuel under pressure)

Gasoline is surprisingly hard to ignite. Gasoline vapor is quite easy. That's why we have fuel injectors designed to atomize the fuel, i.e. turning it to vapor as much as possible. Heck, even in a typical car engine you have unburned fuel - from inside a literal explosion!

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Are centrifugal turbojets easier to make than axial ones?
Looking at the history of jet engines, I noticed that the British first made centrifugal-types, but the Germans went straight into axial-types.
Which type is easier, if any? By that I mean, which type is easier to design and produce? As far as I can tell, they appear to be very similar pieces of rotating machinery, just the compressor blades are shaped differently and the airflow follows a different path.
Are the equations of centrifugal flow somehow much more complex than axial flow, or vice versa? Are the shaft bearings of one much different than the other? Are there tougher geometric problems of lubrication with one but not the other?
Note that I am not asking which is more efficient or the better choice. That would have to take into account much more than the cost to design and build something. For now, I'm only asking about which type of engine is easier to build, if any.

+1 All my career, I have been told that the blades in a centrifugal compressor are much easier to manufacture. I too would love to see a detailed answer.

Centrifugal is easier to make.  It's just a big vacuum cleaner with a kerosene burning furnace behind it instead of an electric motor.  You have a one piece impeller, as opposed to numerous little blades and discs, and can be cast or machined from a block of metal.  It's not dependant on the aerodynamic lift of blades to force air from a large space into a small space. It just pulls it in and flings the air out into a small space, so it's more resistant to surge.  To get surge in a centrifugal compressor engine you have to really obstruct the inlet, whereas with an axial engine you just have to disrupt the flow enough to induce blade stall.
So in the 30s it was WAY easier, cheaper, and faster, development wise, to get a nice running engine using Whittle's design with the centrifugal compressor.  The big downside is that it is less efficient than axial. You can't get the compression ratio that you would get  with an axial because it's difficult to incorporate multiple stages, and the engine is quite large because of the need to fling air out to compress it.  The Germans went with legions of expense, grief and operational delicacy for the better efficiency and smaller profile of axial.
The most obvious example today is the fact that nobody uses axial compressors for turbochargers and most in fact use similar impellers as the turbine as well.  You won't see axial compressors on any of the little micro turbojets used in RC.  A lot of smaller turboprops also use centrifugal compressors, sometimes in combination with axial stages.
That should tell you something about how much easier and cheaper it is to make.

Good point..  shorter.

Centrifugal compressors were common on WW2 piston engines, so jet engine designers could build on this experience. And those centrifugal compressors had heritage in all kinds of liquid and gas pumps.
Axial compressors have the advantage of a potentially higher compression ratio, at the cost of being more complex (more compressor stages).
speculation on my part:
A centrifugal compressor can be machined from a single billet, and the back plate serves to reinforce the individual blades, so it's easier to make a compressor strong enough.
It also means there's more surface area to cool the blades, so you have less problems with the blades melting (on the turbine side).

Regarding cooling, neither axial nor centrifugal compressors need any cooling, as far as I know. I've never heard of any problems/dangers of compressor stages melting except maybe for supersonic aircraft.

fixed that, thanks.

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Could jet engines have a grille to prevent bird or drone entry?
A bird, or indeed any object, entering a jet engine is a very undesirable situation and may cause engine failure. There are also fears we're not far away from seeing the first incident of a drone entering a jet engine.
Why do we not see grilles or cages on the front of the jet engines to prevent this happening?

Any grill that is closed enough to keep bird/drone parts from entering the engine is going to seriously impede the airflow into the engine itself.

While the other question's answers state that this is not possible due to aerodynamic losses, weight, ice collection, etc. yet nobody provides figures to sustain this statement. So the answers could be improved, or this question reformulated into asking for the feasibility (it seems possible) and consequences on the cost of such device.

A grille over the front of an engine will not stop a bird from entering the engine of a plane going at a few hundred knots. It will just stop a complete bird from entering, it will however now be in manageable chunks.
If the aircraft is going slow enough not to mince the bird you will now have a bird sized blockage in the airflow in addition to the surface area of the grille. At least if the bird enters the engine there is still a chance that the turbine can mash the bird into manageable mush.

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Does the canard config have radically different controls than classic config?
Classic configuration is something like the A320: wings in the middle, horizontal stabilizers at the tail.
Canard configuration is something like the Beechcraft Starship: wings at the back and horizontal stabilizers at the front (canards).
I'm just wondering if the canard config has any radically different controls for pitch? Are the directions reversed for the yoke? Are there serious re-training requirements for pilots due to some major difference? Are there bad side-effects when pitching in either direction that must be compensated for?
Note, I'm interested in large commercial jets for subsonic speeds. I couldn't find any in the canard config, so the Beechcraft was the closest example I could find.
Also, I'm asking this with traditional controls in mind...hydraulics, not fly-by-wire. I want to know if there are any radically different pitch considerations, before they are compensated by a computer.
(A related question could be for supersonic deltas, because I know some of them have canards to help stability. But I wanted to focus on subsonic flight in this question.)

The short answer is no, but I'm sure someone will find something interesting to say about some differences.

As far as in-cockpit controls, a canard has the same control setup as a conventional airplane.  Stick or yoke, throttle(s), rudder pedals.
All the differences are hidden in the linkages or hydraulics -- pulling the stick back lowers the elevator on the canard, instead of raising one on a rear stabilizer, but to the pilot it's the same.
There are some minor technique difference in actually flying a canard; stalls are very different (I've heard some canards referred to as "almost impossible to stall"), and trimming pitch is slightly different; correct CG location is very different (usually ahead of the leading edge of the wing or, at most rearward, roughly even with the leading edge of the root of a swept wing), and the engine or engines are almost always located in the rear -- but once again, the pilot manages the engines very nearly identically to a conventional airplane (with the exception that, in case of a failure, he can't just look out the window to see if an engine is on fire).

A canard can never allow the main wing to stall. You are in deep doo doo if it does.  They all are designed so the canard surface stalls and unstalls as it pitches down so the airplane just bobs along with the nose going up and down with the stick full aft, with the main wing never able to get to stall AOA.  It's a safety feature, but also a limitation.

Yep.  Hence "stalls are very different".  As you say, if you manage to stall the wing, you probably won't ever get it unstalled.  I think you could design a canard to allow this (and what happens with a Viggen or Grippen -- can't think those are designed without aerobatic capability), but common ones like Starship or Vari-Eze are designed for safety of low-skill pilots.


text:
For large commercial aircraft, how are the tires changed?
Something at least the size of a 737, all the way up to the A380. How are aircraft tires changed for large commercial aircraft?
Are they actually jacked up like with a car?

In general, the jack is put under the axle of the wheel to be changed. Thus the oleo still carries the weight of the airplane and stays at the same extension, and the jack just has to lift the wheel far enough to be able to get it off the axle. Unlike a car, you don't lift the body off the wheel, you lift the wheel off the ground.

Broadly speaking, yes.  The jacks are larger and more specialised, but the principle is the same.


text:
Can an S-duct fit a turbofan exhaust instead of the intake?
Can an S-duct fit a turbofan exhaust instead of the intake? The idea here is to merge 2 exhausts into one at the exact center of the tail. This would be for twin-engine aircraft that have both engines at the tail, like the Boeing 717.
There are of course S-ducts for intakes, like on the Tu-154. I have never seen them for exhausts, however, probably because it's much harder due to the heat.
I know it is engineeringly possible somehow, because I've seen vectored thrust devices with variable geometry, like the iris types and swivel types. For a large commercial jet, it would be easier because the geometry would be static--no moving parts.
But I want to know if it is economically feasible. To take the heat, I think it just needs some super-pure graphite, which does not sound so expensive to me. But I am not an expert.

Yes they can, and it has been done in the Garrett AiResearch ATF3. Although the engine was not hugely successful, it is still being used in aircraft today.

From this answer:

The hot exhaust stream is deflected back into the fan bypass, which cools it down and results in a low IR signature. This engine is used in the Dassault Falcon.

You can see several S-ducts leading the hot exhaust gases internally into the little staircase-like contraptions behind the fan.

Why is a low IR signature valuable in a business jet?

@Adam for those flying their business jets to the fun parts of the world.

@Adam the engine was developed for a UAV

Not really.  Any type of curve in an exhaust would raise pressure in the duct. Remember a jet engine extracts power from the exhaust stream to turn the fan and compressor as well. Any additional exhaust "plumbing" would be an unnecessary engineering nightmare in addition to adding weight.
But the British do have a liking for stacking jet engines vertically as seen in the English Electric Lightning to eliminate yawing if one failed. This design also significantly reduces drag as the engines are housed in the fuselage instead of in exterior nacelles.
S ducts are ok for air intake provided adequate air can be drawn to feed the engine.  They also may have the interesting property of reducing a 250 knot 15 pound goose into a loose mass of flesh BEFORE it hits the compressor intake by bouncing it off the duct wall, which can absorb the considerable kinetic energy of its impact.

Raise or reduce pressure?

On the exhaust side the "S" would raise pressure in the exhaust duct, there for less pressure drop for exhaust turbines to extract at the engine outlet.

Actually an S-Duct was clause a pressure loss in the jet pipe, reducing the thrust the diffuser can produce.


text:
What costs less energy? Roll or Yaw?
Let's say you want to perform a turn anywhere from 1 to 90 degrees. Which will cost less energy? A bank or a yaw?
Using the rudder does produce extra drag, because the control surface is deflected for a time.
Using ailerons to bank also produce extra drag, but more than that, at a roll angle the lift direction will not be perfectly vertical. You will either lose altitude (that needs to be regained), or increase speed.
That's why I didn't want to phrase the question as "less drag". Lost energy could be in the form of drag, worse lift angle, lost altitude, maybe other things I haven't thought of.
I'm interested in large commercial aircraft as well as "ideal" aircraft with no adverse-torques (all control surfaces are aligned with the center of mass).

I think the correct answer is to perform a coordinated turn using both rudder and ailerons, but I'm sure someone will answer with a more satisfying mathematic equation to explain why that is so.

Surely the least-energy turn is the one which doesn't make passengers spill their drinks, and ideally are unaware of the turn at all unless they're looking out the window.

When you turn by yawing you are skidding the airplane to point the nose to the side, to offset the trust vector to move the plane sideways as it's going forward.  The rate of turn you can achieve this way is very low and there is a massive drag of being in a skid as you slither around like a car on ice with the fuselage side presented to the airstream.  To the extent that the airplane has a roll/yaw couple (roll caused by yaw) you will have opposite aileron to keep the plane from banking into the skid; more wasted energy serving no purpose.
When you turn by banking you offset the lift vector to move the plane sideways as it moves forward.  There is no sideslip so the drag is only increased by the small amount caused by the increased elevator and AOA, and the minor drag of the moderately displaced ailerons and rudder (to the extent they are displaced at all once the rolling action is complete).
Because you are moving through a fluid, a skidding turn is like turning a car by drifting it on a dirt track, a banked turn is like going around on a banked corner, where you don't even have to turn the steering wheel to keep aligned with the road as the car changes direction.  Which way is more efficient?
To find out I tried it on my own plane, a homebuilt PL-2. I flew it at 2000 ft, holding 70kt, close to max L/D for this plane, and applied full rudder with a bit of opposite aileron to maintain wings level (not much aileron is required on this plane), adjusting power to hold altitude at 70kt in the skid.
My plane has a manifold pressure gauge even though the prop is fixed pitch, useful for knowing precise power settings. It took 19" MP @ 1900 RPM to hold a skidding turn with full rudder at 70 kt and the turn rate was about 90 deg in 30 sec, or about Rate 1.  This is about 55 HP on the Lycoming O-290 D2.
I then took out the rudder and banked into the turn (about 15 degrees bank was required to maintain Rate 1) to maintain the same rate of turn as the skidding turn but in a coordinated bank.  To keep from climbing I had to reduce power.  I ended at at about 16" MP @ 1700 RPM, in a banked level Rate 1 turn at 70 kt, which is somewhere around 45 HP.
So it took somewhere around 20% less power to maintain a banked turn at Rate 1 than a skidding turn at Rate 1 when flying near max L/D.  The extra drag of the skid was also quite obvious in the deceleration when applying the skid with rudder vs rolling into the coordinated turn.
With airliners, turning by skidding is out of the question because with swept wings you get a powerful roll rate as soon as you yaw a couple degrees and you will quickly run out of opposite aileron trying to keep the wings level.

A quantification would be good.

Well that's a project in itself.  "Way more drag"  and "much crappier turn" is sufficient quantification for something like this, it being pretty obvious to anyone who flies.

I disagree. They sound like statements based on how it feels to the inner ear. Your banked car turn provides all the extra lift required at no extra power.

Actually the car is under increased load from the G of its turn on a banked track, so tire compression and rolling friction is increased.  Anyway, you would have to go out and do flight testing to determine turn rate X with side slip Y (and believe me even with full rudder it isn't much), measure the drag increase, and compare that with a banked turn of the same rate, in the same airplane, same conditions.  I can tell you that the differences are obvious if you actually go do it (and I have played around with turns by skids in the past just for fun), so obvious that measurements are redundant.

+1 for the testing.

@Koyovis Thanks.  Drag penalty aside, I must say I was surprised by the turn rate I was able to get by skidding.  I'd never tried it in a sustained way before.  Also  some of the extra power may have been to compensate for the prop working while slewing sideways, so perhaps some of the drag penalty actually thrust efficiency loss.  Out of curiosity, I'll try it again while gliding power off to compare the sink rate of coordinated turn vs skid.

A constant turn requires a constant centripetal force, i.e. perpendicular to the direction of motion. This force has to be generated aerodynamically (unless rocket engines are used).
An aerodynamically generated force is always accompanied by induced drag - even if the force is generated by the fuselage (due to sideslip) instead of a traditional airfoil. For minimal energy expenditure, you want to minimize the induced drag for a given aerodynamic force. In other words, you want to have a high lift/drag ratio (L/D).
For virtually all heavier-than-air aircraft, a large part is dedicated to fighting the force of gravity (either fixed wings or a rotor). The surfaces used for fighting gravity are thus invariably the surfaces with the highest L/D.
By rolling the aircraft, the lift generated by these surfaces is tilted inwards, and it gains a force component perpendicular to the direction of motion. Rolling thus uses the most efficient means of generating an aerodynamic force - the wings or a rotor - to generate the centripetal force.
It gets better, still. Even if you had a fuselage that somehow had the same L/D in yaw as your fixed wings, a roll is still more efficient. This is because a single tilted force always has a lower magnitude than the sum of the horizontal and vertical components per the triangle inequality.
Use yaw to coordinate your turns, and use roll to do the actual turning. In a coordinated turn, the most efficient speed may be somewhat higher because the balance between induced drag (reduces with higher speed) and parasitic drag (increases with higher speed) is shifted. Compare this with a glider, which has a higher speed for optimal L/D (optimal range) with increased ballast.

Note this is not a violation of "pitch controls speed, power controls altitude", it's more like extra AOA is more efficiently added to turn, power is added to compensate for additional drag while turning.  Now we can examine "should we maintain optimal wing AOA and speed up in a roll and turn, or increase wing AOA to a lower L/D, but not speed up as much (maintain airspeed)"?  Due to increase in parasitic drag, it may be most efficient to add only enough thrust to maintain airspeed, while using the wing and rudder in a coordinated turn.

The roll method is the most efficient.  It requires only a small amount of extra lift from the wings to sustain level flight in the turn.  Yawing puts the fuselage at an angle to the wind in order to provide "sideways lift", and produces much more drag in the process because the fuselage is not shaped to provide lift efficiently.  Indeed "side slipping" is sometimes used as a way to slow down in an emergency.

@Sanchises Never mind mate, I edited my answer, should have done that in the first place.

@Koyovis No problem!

@RobertDiGiovanni "Pitch controls speed, power controls altitude" is a teaching aid, not a rule in physics (basically, it is to make sure the student pilot adds power proactively instead of retroactively). I addressed the rest of the comment, regarding optimum speed, in my answer.

+1 for your answer, and somehow that slow wing is making 4x the force as the spinning prop for many GA aircraft.  So it certainly has some to spare for turning, as long as we keep an eye on that AOA.

Depends on the speed: it costs very little extra energy to yaw a hovering helicopter.

Yawed turn. A yawed turn increases sideslip angle $\beta$, creating a sideslip velocity which results in an aerodynamic force $Y = C_Y \cdot ½ \rho V^2 S $. Picture above is from measurements of an F-27 model with no flap deflection $\delta_f$, and shows an almost linear relationship between $C_Y$ and sideslip angle. The lift vector stays pointing straight up in a purely yawed turn, no additional lift required.
Banked turn. The lift vector points into the turn, meaning some of the vertical force is lost. Angle of attack needs to be increased for the extra lift, creating more induced drag.

That is about the force vectors, the question is about energy = force * distance or force * velocity * time, and that is where an exact workout becomes laborious. This data is available from simulator aerodynamic packages, but there are limitations on data reproduction.
Qualitatively speaking, the difference between the two is:

Drag due to sideslip in yawed turns is a parasitic drag, and increases quadratically with airspeed;
Induced drag reduces with airspeed.

So at low airspeeds, a yawed turn requires less additional power. At higher airspeeds, a banked turn requires less additional power. The crossover speed may be quit low, intuitively we would tend to believe that above stall speed a bank wins.
Of course, the above does not mention the comfort zone, our brains and bodies are way more comfortable experiencing normal forces than they are experiencing side forces.

Angle of attack needs to be increased for the extra lift, creating more induced drag. I thought airspeed must increase  in order to maintain enough lift, or else you sacrifice altitude. Even if AoA alone is enough, it would seem to require more engine power at the higher AoA. Either way seems to be adding energy from engines. Btw I always figured that pulling back on the stick during the bank is only to make the turn go faster. If you dont pull back, the bank will still accomplish the turn.

If you fly trimmed, then bank without pulling back, the vertical force is lift * cos $\beta$, so diminishes with bank angle. Vertical force is less than weight and the aeroplane descends. Pulling back on the stick increases the lift vector, and no more altitude is lost when L*cos$\beta$ = W. Indeed, the horizontal component is now larger as well and the turn is tighter.

Are you really suggesting that when flying near, say, the speed for minimum sink rate or max climb rate, you can turn most efficiently by skidding rather than by banking?  If this were true, it would be the preferred strategy of glider pilots circling in a thermal updraft.  This is not the case at all in reality-- in fact there is an argument that a slight amount of slip is helpful while circling in a thermal updraft.

@quietflyer no I'm not setting the transition point near minimum sink rate or max climb rate, merely using the graph as an illustration of the different nature of parasitic and induced drag.

@DrZ214 - Re other comments above- yes you can turn without pulling the stick further back.  The aircraft will go through a pitch "phugoid" and then stabilize at an increased airspeed due to the extra wing loading from the bank angle.  And yes you certainly can hold airspeed constant as you roll into the turn, if you wish-- by moving the control stick aft.  Either method-- constant airspeed or constant fore-and-and-aft stick position-- will require increased power IF you wish to hold a constant altitude in still air.  Which will require MORE power depends on what part of the flight envelope

@DrZ214 - ctd - Which will require MORE power depends on what part of the flight envelope you are in.  If you are flying way above min sink rate speed, and you are adjusting the power as needed to hold the altitude constant, then holding the airspeed constant will require less extra power than allowing it to increase, and forcing the airspeed to decrease will require still less extra power.  If you are initially flying fast enough in wings-level flight, then with enough airspeed decrease (back pressure on stick) you can sustain banked, circling flight without adding ANY extra power at all.

@DrZ214 -- ctd -- in fact in a typical light plane, with fixed-pitch prop, if initially flying fast enough in wings-level flight, it is easy to demonstrate a transition to circling flight, with no loss of altitude and no change in throttle position.  The key is sufficient back pressure on the stick/yoke.  The angle-of-attack in the turn will be greater than in wings-level speed, and the airspeed in the turn will be less than in wings-level flight.  The engine will actually be putting out LESS power in the turn, because the RPM will be lower.

@DrZ214 -- ctd - this only works because as the plane enters the turn, the wing is moved to a much more efficient angle-of-attack than it had in wings-level flight.

@quietflyer which means that the plane has lost kinetic energy.

@Koyovis --true-- in case it is unclear, I am not so much focussing on the transition (time when airspeed is changing) as on the final steady-state case after the airspeed has stabilized.  When the airspeed is changing (decreasing), this energy is converted into potential altitude.  So let's say you start in wings-level flight at 120 mph and end up in a steady-state banked turn at 80 mph with no change in throttle position or altitude.  As you were decelerating you could TEMPORARILY maintain altitude at 110, 100, 90 mph as kinetic energy was bled off and converted into potential altitude.

@Koyovis -- But only at 80 mph was steady-state flight possible at the established bank angle and throttle postion-- now you are no longer dependent on the kinetic energy "released" by the deceleration.

Sorry, too many words.

@Koyovis I see an equation for the Yaw case, but not the Bank case. Is there one? If you can add it, then we could compare them directly mathematically. BTW can you also explain that chart a little better? I see terms like delta_f and beta (degrees) but dont know what they mean.

@Koyovis You have to associate a given yaw angle with a turn rate, and compare that total drag in that condition to total drag at the same rate of turn in a banked turn and you'd have to go out and test it.  The yaw angles required to get anything close to a decent turn rate are substantial.  Next time I'm out in my plane I'll try to do that.  I'll see what turn rate I can get with a flat skidding turn at a constant altitude and a constant 80 kt with full rudder, note the RPM/MP, and compare the power setting required to meet the same conditions in a banked turn at the same turn rate.

@JohnK Great suggestion!

@Koyovis Thanks but in the equation Y = Cy...S , what is S? About force vs energy, that is okay. If we have force equations, we can just look at which force is less. But I still don't see an equation for case 2, the bank. Is there one?

@DrZ214 S is the wing surface area, which the $C_Y$ measurements in the graph are scaled to. Yes there is an equation for bank centripetal force = $L * sin \phi$ with $\phi$ = roll angle. So in that case the question would morph into: which method of turn creates the least drag over the time of the turn? It does make sense to include some measure of duration or heading change angle in there.

@Koyovis  So I did a rudimentary test in my own plane yesterday and updated my answer.

The best way to look at this IS to look at drag, that is what "costs you energy".
We have arms and legs.  Why don't we walk around on our arms?  It's theoretically possible!
I could even write an equation for It!  Let's try some common sense first.
Turning requires motion, as does heavier than air flight.  There for the hovering case cannot be considered, or any variable airspeed.  Let's look at flight at a given airspeed.  In ALL cases lowest drag is most desirable.
Assuming no change in altitude or airspeed executing the maneuver, how do we do it with the lowest drag?  Certainly not by turning the fuselage sideways into the airstream and using the engine to pull us in a new direction.   Banking the aircraft uses the far more efficient horizontal component of the wing lift vector to create side ways motion.  This is the "turn".
Coordinating the turn with rudder keeps the thrust vector and empennage "following" the new direction of flight, with the best use of thrust vector, and least amount of drag, as possible.
Breaking the turn down to many instantaneous steps can help clarify the thought.  If you are worried about that huge wing creating drag by moving side ways,  have a good laugh.  At any given instant it is accelerating horizontally from V zero (very little drag) while maintaining the vertical lift component. This is why turning is considered acceleration, even though airspeed is constant.
Comparing the graceful slow wing to a furiously churning prop, few could argue for the latter.

-1 for dubious assumption. Assuming no change in altitude or airspeed executing the maneuver... This is impossible in a bank. If your lift vector is tilted sideways, you will sink or increase speed to increase the vertical lift component to match your weight. This is precisely why I phrased it "least energy" and not "least drag". In a bank, you could just sacrifice altitude. Your speed is the same so there's no extra drag from that, but you lose energy due to decreased altitude.

Whoops, no I see what you're saying now. Increase AoA but at the same airspeed. This increases lift so altitude can stay the same too. And ofc higher AoA is higher drag too. +1 instead...except my vote is locked :-( New SE rules maybe?

@DrZ214 No, no new rules. Once voted you have 5 minutes (?) to change your mind but if the post is edited you can again change the vote. Maybe fix some minor typo (like "there for" vs. "therefore") yourself and then re-vote ;-)

My fault for not being clearer.  Wing or prop can change direction, but only prop can add energy to the system.  To maintain speed in an extended turn you add power, to maintain altitude and turn you add AOA.  Rudder coordinates, savings is  from wing being more efficient to accelerate side ways unless ..... the wing and prop were the same size airfoil like .... (pause for drama)  .... birds!

"We have arms and legs. Why don't we walk around on our arms? It's theoretically possible! I could even write an equation for It! Let's try some common sense first."  Best answer I have seen in a while!  So many questions from non-pilots, and so many complex explanations of very basic principles on this site, it boggles me...

@MichaelHall Different people come with different interests. Some are interested in how to fly a plane, some are interested in how a plane flies. Sometimes questioning what seems obvious may lead to new insights, which seems like an excellent use of a Q&A website.

@Sanchises - I understand and agree.  I guess my critique is that answers are often disproportional to the complexity/demonstrated knowledge level of the question.  There are some brilliant people who post here, but not every question warrants an engineering thesis to explain.


text:
When is VOR navigation mostly used?
What pilots use VOR navigation: GA pilots, commercial pilots, military pilots? When is it most used?

It may be useful to note that the VOR system is being (slowly) decommissioned. Mostly by not servicing the VOR stations anymore, when they stop working, they are decommissioned. Some VOR stations are being maintained, but the aviation system in the US is moving towards NexGen. VOR's are used by all the pilots you listed, and I'd say a majority of them are IFR pilots.

@RonBeyer It's important to note that the VOR system is not being decommissioned, it is being downsized to a "minimal operational network" (which is admittedly a major reduction from what's currently available). Ground-based navaids like VORs are expected to remain a part of the national airspace system for the foreseeable future. This slide deck is a bit dated, but the general information is still current

@voretaq7 Decommissioned isn't the right word there, its being reduced to a "minimal navigational network" and giving navigational priority to GPS, keeping the VOR stations around in case of "widespread navigational satellite outage". While they may not have plans to fully decommission the VOR system, hanging on to it for backup to nav-sats is going to become less of an argument considering there are 2 different constellations (GPS and GLONASS) and two more in development (Galileo and BeiDou).

@RonBeyer This is getting into chat territory, but the reason they're hanging on to ground-based navaids (VOR, ILS) is that the failure modes for all satellite-based systems are similar: Multiple constellations and redundant satellites can't eliminate the catastrophic common-mode failures (ionospheric disturbance, solar flare, etc.), they just fix the "Oh crap, we lost a satellite!" problem.

the last time I used VOR for primary navigation was on my IFR checkride

On military airfields you usually find a TACAN instead. It's similar to VOR/DME (gives radial and distance), but more precise. VOR approaches are still used, especially when ILS is under maintenance.

Im not sure on military but its still used by GA and Commercial pilots frequently. Although as has been noted in the comments it is slowly being downsized. It should also be noted that Commercial planes and GA planes use the same facilities and the same ATC. Once in the air a C172 may be given the same instructions as a 777 with the only differences being altitude and speed.
On one hand some VOR's are used for approaches to some airports. There is an effort to replace these with GPS approaches but there are still active VOR approaches out there today so anyone flying IFR to these airports needs them.
They are used for all sorts of IFR navigation and are often the terminal points on an airway, so you may be cleared after departure direct to a VOR.
VOR's can be used as a fixed point for a hold in IFR flying.
VOR navigation (and the use of paper charts for that matter) is still taught during PPL training since GPS is not necessarily installed in every plane.
Some FMS units use RNAV as a backup to GPS or inertial navigation.
This AOPA article covers most of their uses.
Personally, I am a big fan of GPS and fly direct when the airspaces and clearances allow for it (VFR). That being said, the planes I rent generally have dual Nav/CDI installations which I frequently use to cross check my position with the GPS.


text:
What is a Fuel Over Pressure Protector?
What is a fuel over pressure protector on the A320 and other aircraft? What is the purpose of this hole?

I think the subject of this question is somewhat related (but not duplicate).

It is possible to overfill a wing. You fuel the aircraft when its wing is cold. Later as the aircraft sits on the ramp the outside temp increases causing the full to expand in an already full wing. So they have the overflow system dumps the excess fuel on the ramp. Its usually not a lot of fuel just a few gallons.
  It also prevents the fueler from over fueling the wind. It can sometime dump 50 gallons or more if the fueler it not paying attention.

I was recently on an MD-88.  As soon as the door closed the captain came on....Always a bad sign.  He said that he had just been told of a fuel leak.  It turns out that the fueler had overfilled the aircraft and it had vented.   We had to wait around for a maintenance check, a cleanup crew, a ramp approval and a fire department approval (hazardous waste spill?) before we were finally able to depart.


text:
What types of antennae do aircraft have and what are their functions?
What types of antennae do aircraft have and what are their functions? Examples are HF-, VOR- and SATCOM-Antennae.

This list here should get you started

This is a very broad question, and the only real answer is "There is an antenna for every transmitter or receiver the aircraft will need" -- If an aircraft needs to transmit or receive shortwave someone will cut a hole and mount a shortwave antenna on it. If it doesn't need that capability it won't have that antenna.

Unusual antennas on an AWACS. Else this description should help in the identification, with the related pictures.

Like everything in aviation this depends on how the plane is configured and what equipment it contains. This question is pretty broad and there are even some non aviation related antennae on modern commercial planes for things like onboard internet but lets cover the aviation related ones. AOPA provides a great document on it here.
Any plane equipped with GPS will have a GPS unit somewhere on the airframe (usually on the top)

(source)
There are generally 2 radio antennas for the communication radios. Traditionally one is mounted on the top of the plane and the other on the bottom.
On smaller GA planes they generally look like this

(source)
If your plane is equipped with VOR receiving capability you will have a VOR antenna as well which is generally mounted on the tail.

(source)
You will also have a UHF antenna for your transponder and DME, its located on the bottom of the plane

(source)
If equipped with an ELT you will also have an ELT antenna, generally on top.

(source)
It should be noted this is the general setup of most smaller GA planes. Large commercial planes have the same general equipment but often have greater redundancy and/or more units than your typical GA plane. Some airframes also have antennas for systems that are being phased out like Loran based navigation (which I think has been all but shut down now).

There are also blade-style transponder antennas (and DME antennas), combined VHF/GPS antennas, marker beacon receiver antennas, ADF Sense & Loop antennas, etc. - The antenna section of an aviation supply store has a bunch of types to look at.


text:
What are the benefits of using Krueger Flaps?
I mean why using Krueger flaps in some aircraft instead of using slats all over the leading edge?

On a related note, the Krueger flaps on Boeing's old test 757 (NA001) worked well to shield the vast majority of bug hits from impacting the leading edge, therefore not effecting the upper wing laminar flow goals of the NASA funded HLFC (Hybrid Laminar Flow Control) program.

There are different types of Krueger flaps used, with increased efficiency coming at the cost of increased complexity.

The simple ones are basically flat panels that folds out from the wing lower surface into a very steep, deployed position and seals against the wing leading edge; though these are extremely simple, they don't offer much in the way of stall protection.
The next type has large folding 'bull' nose that offers increased performance at the cost of slightly increased complexity. This type of flaps is used in B727, B737, and inboard of B747, as shown in the figure below:

By Arpingstone - Photographed by Adrian Pingstone in August 2002 and released to the public domain., Public Domain, https://commons.wikimedia.org/w/index.php?curid=118310

The most complex type of Krueger flaps are the variable camber type, which has a flexible panel (which forms a slot at the trailing edge) and a bull nose. This provides a better $C_{L_{max}}$ performance compared to a slat, but gives poorer takeoff L/D performance compared to the slat. However, the main problem is that the mechanism is exceptionally complex, as can be seen in the figure below. As a result not many aircraft use these.

Image from aviation.stackexchange
This means that simpler Krueger flaps are used in the inboard with slats in the outboard region, so that the flow remains attached in the outboard regions at higher angles of attack compared to the inboard region.

Many large airliners, particularly Boeings, use Krueger flaps closer in to the fuselage and slats further out on the wing. This is primarily because slats resist a stall more effectively, so by placing Krueger flaps on the forward parts on a swept wing and slats on the rearward sections, it encourages a more balanced, controlled landing stall. Another reason to use Krueger flaps is that they are lighter, both with the equipment itself and with its actuators, and can be used on thinner wings that are too thin for slats.

Another purpose of the krueger flaps is to promote "wing root stall first before the wing tips" concepts at high angle of attack. They create a turbulent air stream over the wing which directed towards the tail plane and create an artificial buffetting to make aware the pilot that aircraft is leading to stalling.


text:
What are these circles below the wing?
There are some circles below the wings of these two aircraft. Look:

Source: Airliners.net (modified)

Source: Airliners.net
On the second picture the circles are clearly visible on the wing. What are they? What is their purpose?

Clearly, they are not part of a collision avoidance system.  Ouch!

They look like inspection ports

They are inspection ports as @DanPichelman says.

I was prepared to say "engines", but I guess you're talking about different circles... :)

In the US, those are called either "Access Panels" or "Inspection Covers", depending on their function, but many people combine the two terms and call them "Inspection Panels". Based on the comments, other parts of the world apparently refer to them as "Inspection Ports".
We remove both panels and covers to perform routine inspections, but access panels cover system components that may be accessed more often for servicing. Both panels and covers are designed for quick removal, typically fastened with screws, as is the case in the aircraft depicted in the photos above.
Here is an inspection hole with the cover removed. This is on a Cessna 170 that I am in process of rigging. Note the aileron cable turnbuckle visible inside the wing. Through this access panel I will adjust the cable tension, among other things, and once the cable is properly tensioned I will safety wire this turnbuckle. Here the panel is removed for maintenance, but during an inspection I would remove this panel to inspect these same systems.

These are inspection rings. From FAA Advisory Circular 43.13-1B (complete PDF is here):

Inspection Rings. Inspection access is provided adjacent to or over
  every control bellcrank, drag-wire junction, cable guide, pulley, wing
  fitting, or any other component throughout the aircraft which will be
  inspected or serviced annually. They are installed only on the bottom
  side of the wings except where installed on the top surface by the
  original manufacturer.

This article also mentions it from a student's point of view.

I'm sorry, they are not. Inspection rings are used in fabric aircraft construction, as you cited from 43.13 Chapter 2, which is the chapter on Fabric. Inspection rings are used to create inspection access on fabric covered aircraft.


text:
What are the RVSM Critical Area markings?
I couldn't find any image to show you what I'm talking about but on many aircraft there are markings around the static ports and the pitot probes that say RVSM Critical Area. What is this?

Aircraft need to know their altitudes to avoid bumping into each other in the vertical dimension. The default instrument for this is a pressure altimeter. A pressure altimeter needs access to a static port.
RVSM stands for Reduced Vertical Separation Minima. Basically, aircraft above a certain altitude (29,000 ft) used to be kept further apart from each other vertically (2000 ft) since the accuracy of the pressure altimeters degrades with altitude.
Over the years, as traffic density increased there was a need to pack aircraft closer vertically (1000 ft). This is possible if you have a special more accurate altimeter system in all aircraft and these ops are RVSM (late 1990's and early 2000's).
Now, in order to guarantee that this system works as designed several points need to be satisfied one of which is the integrity of that static port I mentioned earlier. Static ports can be sensitive to external air flow and hence any variation / disturbance / abnormality in the area immediately adjacent to them on the fuselage must be avoided whenever you want to use the aircraft for RVSM operations.
And hence the markings to serve as a guide to all involved in aircraft ops about which is the area to worry about. e.g. Any imperfection in that region will have to be dealt with immediately. And worst case the aircraft would not be capable of flying in RVSM operations till any problems are fixed.

The RVSM critical areas must be inspected prior to every flight to ensure they are free from debris, deformation, or other damage. This is to ensure the altimetric accuracy of the pitot/static system for operation in RVSM airspace. This inspection is typically done by a pilot during their pre-flight walk-around.


text:
What is this little door on the upper deck of the A380?
I found a little "door" on the upper deck of this A380:

Source: Airliners.net
What is the purpose of that door, hatch, whatever?

At a guess a ladder for the emergency exit.

Looking at Airbus's diagram gallery on the A380, this image suggests that the panel is to store and deploy the emergency evacuation slides.

This is also supported by another picture from the same document, in which similar doors are seen under each second row entrance.

Those planes are beasts!!! Saw one at an airport I connected through last week.

It's for the emergency slide.
You can see it deployed here:

Source:  Airliners.Net

It seems only the upper deck slides have a separate door (I assume it's because they don't fit at the normal location in the door). But I don't see the difference in the picture.

@mins, I don't know, but the inward slant of the upper deck door might have something to do with it.

@JanHudec: From this book, the reason is about the size of the slides (length and width). Also as the slides are used as rafts, they need to adapt to the 700 passengers need.

See video of slide deployment

On the bottom deck the slide is packed inside the doors, but on the top deck the are in a small compartment underneath the door.


text:
What are the advantages and disadvantages of a monoplane, biplane and triplane?
What are the pros and cons of a monoplane, biplane and triplane design; aerodynamics, efficiency, etc.?

Related: Can biplane or triplane designs be revived with modern materials? and also Why are there no longer any biplanes?

That is a very broad question—though I'm not sure that it is quite broad enough to close.

One way to look at the differences between them is by looking at the effects of having multiple wings.

Having multiple wings means that the same aircraft can produce more lift for the same wingspan, as they have a greater wing area.
As a result, the biplanes and triplanes have lesser wing loading, which results in shoter to/landing and lower stall speeds.
The shorter wingspan results in reduced roll inertia and roll damping, which results in better roll rates in the bi- and tri- planes. This improved maneuverability was the reason for the surge in triplane designes during WWI.
Due to interference effects, the bi- and tri- planes actually require higher wing area to produce the required lift. Also, the drag is also higher than the monoplane, which gets more pronounced as the speed increases.
The main reason for the use of bi- and tri- planes was that they provided the required strength for the type of airfoils used. As high strength materials became available, this mattered less and the aerodynamic disadvantage of the biplane (and triplane) designs came into fore.

The aerodynamic advantage of the monoplane along with improvements in materials used has made the monoplane the de-facto standard except for certain specialised applications (like low speed aerobatics)


text:
What are these lines on the nose of a Boeing 737?
What are these lines on the radome?

You can also see them here:

What are they and what is their function?
Source: AviaFilms

I'm no expert on B737 radomes, but I was able to find that these are called Lightning Diversion Strips or Lightning Diverter Strips.
Since radomes are generally built with composites, they do not transmit static electricity in the same way that aluminum skin would.
The B737-200 maintenance manual states that the radome is constructed of fiberglass and is provided with lightning diverter strips on the outside surface—though it states that these are not necessarily provided on all airplanes. It further states that these diverter strips are constructed of thin gauge aluminum.
From b737.org.uk:

The radome (RAdar DOME) is an aerodynamic faring that houses the weather radar and ILS localiser and glideslope antennas. Unlike the rest of the fuselage it is made of fibreglass to allow the RF signals through.
Fibreglass is non-conductive which would allow the build up of P-static (static due to the motion of the aircraft through precipitation). This would in turn cause static interference on the antenna within so the radome is fitted with six conductive diverter strips on the outside to dissipate P-static into the airframe.

And from this article:

Another important characteristic of the radome is its ability to bleed
  off static electricity. This is done through the use of
  anti-static/anti-erosion coatings and through the lightning diverter
  strips extending from the frontal area of the radome to the airframe.
  "These diverter strips, says Bauer, continuously collect static
  build-up from the surface of the radome and conduct it to the airframe
  without severe sparking or arcing."

Here is a video that explains some of the principles of lighting diversion (note that the particular strips discussed may differ from those found on the B737). The diverter strips are discussed particularly starting here at 2:20:

The lines on the nose radome (radar dome) of airplanes are called segmented lightning diverter strips, and they're made by a few different companies, one of which is Weather Guard Lightning Tech
Electric charge builds up on the nose of the plane (on the radome) as makes contact with high-velocity particles in the air, which include rain, ice crystals and snow. This charge buildup creates streamers, which reach out into the air and, of course, are how lightning strikes originate.
The charge in clouds will being streaming down in a stepped leader, and will eventually meet and connect with the streamers on any object, which includes a plane.
So, the lightning diverters on a plane's nose are metallic, highly conductive strips that are where lightning streamers originate, so that if a strike occurs, the lightning energy is directed down the strip to the metal frame of the aircrafr rather than puncturing the composite and non-conductive radome. Lightning diverter strips save the equipment inside a radome from lightning damage, and the segmented button structure causes the lightning to jump from segment to segment in the air, thus creating less heat onto the radome surface itself. You'll find more info in this article as well: https://weatherguardaero.com/what-does-an-aircraft-lightning-diverter-do/

authors: Алексей Евтушенко


text:
Is it unusual for an aircraft to flash its landing lights in sequence?
Walking this evening I looked up and saw a plane which I assumed was on approach to Liverpool John Lennon. When I looked carefully I could see that as well as the anti-collision strobes it was flashing two lights in sequence. I assumed they were the landing lights as they were very bright and positioned towards the wing tips. However, they were clearly flashing one at a time, left then right.
I've never seen this before, is it normal and does it signify anything?

Related: Do lights on aircraft flash in a specific pattern? and some others.

The flashing lights help to make the airplane more noticeable. They have also been suggested to reduce the risk of bird strikes. This is offered as an option or add-on for many types of aircraft.
In addition to helping the pilots to see ahead of them when on the ground in dark conditions, landing lights help to make planes more visible to others. The FAA requires landing lights to be on for commercial flights under 10,000 feet, and encourages pilots to turn them on at least when near an airport or in low visibility. Flashing the lights makes them less likely to be confused for something like a star at night.

Thanks for your response. I was out with dog the  before and realise I often see aircraft blinking their landing lights. What stood out was how slow the sequence was, it was more like the flashing of police car headlights. Still, if they're there to get noticed, that's what they did.

It is certainly a standard feature on the Dassault Falcon, we have a "pulse"position where it flashes as you describe which we switch below 10 000 ft, and then the "ON" position which we use on we are clear to land.The latest being purely a company SOP.

I thinking what you saw were the landing lights. Airplanes do tend to have taxi lights and landing lights in order to make it easier for pilots during night time.

Yes, that was my thinking, but I've never seen them flash left-right, left-right in sequence.


text:
Destiny of Unused food after flight
What happens to the unused food after a flight? Does it get all destroyed (on-site or off-site) or is there some alternative use (e.g. Charity ) that it gets put to. Does the incoming food truck cart it off with the carts or is it handled by the Janitorial / Clean Up crews.
On shorter sectors can some items be used on the next flight with the carts only being "topped up"?
PS. Obviously, I'm referring to the perishable items with short shelf lives. Not talking about peanuts and other packaged items that last with longer shelf lives.

What if the garbage (on international flights) contains plant or animal products banned from the destination country for disease control reasons?  Is the airline forced to carry their trash home with them?

I will say about airlines in Brazil.
The meals that are not eaten by passengers, after the flight, they are loaded off by the catering service responsible for this, and they are tossed.


text:
Which Aircraft Models need a Tripod Jack for tail support at airport?
Which are the aircraft models that need their tail to be supported by a jack when empty?
Wikipedia says this:

Aircraft Tripod Jack
They are used to support a parked aircraft to prevent their tail from
  drooping or even falling to the ground. When the passengers in the
  front get off an aircraft, the aircraft becomes tail heavy and the
  tail will droop..... When needed, they are tugged to the tail and
  setup by manpower. Once setup, no supervision to the jack is needed
  until the aircraft is ready to leave.

Are there even commercial airliners that need to be supported this way every time they load / unload? Sounded like a lot of work for ground staff. Was this a legacy equipment thing or still in use today?
https://en.wikipedia.org/wiki/Ground_support_equipment

Related

Anecdotally, Dad and some colleagues on a business trip got a chance to look round Concorde in Paris : the guard counted them; six; and asked them not all to go to the rear of the cabin at the same time...

The Quest Kodiak uses these.

The Cessna Caravan also uses tail supports.

Tripods are mostly found in cargo aircraft (and combis) as the cg shift is severe there (particularly for front loading). A good example is the Boeing 747s. Also used in DC10/MD11s used as freighters.

Image from reddit.com
Some 747-400 combis also use these, though it is mainly for cargo loading.

Photo: Je89 W. / Source: PlanePictures.Net
Some passenger airliners are prone to tipping (737-900 comes to mind) and are usually supported by a pole rather than by a tail stand (same procedure is used in a KC-135 tanker too). For example, a Boeing 737-932ER with tail support.

Image from airliners.net
The tail supports are not strictly necessary and the loading can be carried out sequentially (from front to back) for safe operation, but are used as a  precaution. In some aircraft (like ATR-72 and Saab 340, called pogo sticks) small tubings are attached to the rear fuselage to prevent tipping over when loading/unloading.

Image from theflyingengineer.com
The nose can be tethered to prevent tipping during loading/unloading as done in this Korean Air 747F

Image from koreanair.com

Also in some older aircraft such as 727 the airstairs performed this function.

Are you sure that EVA 747-400 is a combi?  The passenger cabin seems to extend awfully far back for a combi aircraft...

@Sean how do you see to where the passenger cabin extends? Afaik all 747 combi's have windows that extend all the way to the back with a (moveable) bulkhead in between to separate the cargo and passenger space. The aft cargo bay door is on the other side and not visible in the picture.

Your 737 link is dead. Looks like someone didn't pay the bill.

DC-8 freighters used tail supports as a safety precaution. As I remember, if you loaded them properly the support wasn't needed, but people didn't always do that. The cargo door was forward of the wing. I've long since thrown out my DC-8 weight & balance manuals, but as best I remember, the proper protocol was to put the desired most forward ULDs in first and slide them to their positions forward of the door. Next you would put in the rearmost ULD, but then move it just far enough aft to get the next ULD on, then move it and the previous one just enough aft to get the next one on, and so on. At some point you had enough weight forward of the main gear to start moving pallets all the way to the rear. If you miscalculated, the airplane would sit on the installed tail support. This reportedly happened at Newark, as I remember, in the early 1990s, but they had neglected to put up the tail support. The airplane sat on it's tail.
The same attention to balance was also needed when unloading. Read this link for an unloading accident when the tail stand collapsed, causing one of the cargo pallets to roll down to the tail of the cargo compartment.  Three loaders were able to get out of the way of the runaway pallet, but the fourth got his foot caught, sustaining serious injury.

Edited to clarify what, precisely, injured the fourth loader; the way it was worded previously made me originally think that he was injured when the DC-8's tail sat on his foot.

I work for UPS, we load A300's almost every day. It is the only aircraft we use the tail stand for. Across the airport at FEDEX, they also get A300's but they dont use the tail stand, So i guess it depends on the company.

Back in the day I was weight and balance / loading qualified for all B747 types. The tail-stand is promarily used for freighters. If I'm doing a full load change (eg all off and reload) with a crew familiar with the aircraft .. , then I wouldn't be too worried about tipping a freighter. BUT for a 747 Combi I would say that a tail-stand is indispensable! Especially for a transit flight where everything is happening all at once.
A fully loaded 747 combi and you have to have your wits about you when working around this aircraft.
The MD11 freighter was another aircraft you had to watch out for. An empty plane will have its CG (centre of gravity) right up the back but once fully loaded the CG will be at the forward limits! To get it in trim you have to load all the heavy stuff back in the rear of the aircraft but this will require a careful loading sequence as the maindeck cargo door is in front.

Aircraft of any type should be loaded
From the front cargo hold .. tripod must
Be fitted to rear of aircraft .
The loadmaster must carefully step by
Step take the right procedures .
Once certain amount of load is put in
The front cargo hold a thumps up is
Indicated to rear FMC driver to commence
Loading .

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Your first sentence seems to imply that for aircraft of any type, a tripod must be fitted to the rear of the aircraft.  If this isn't what you mean, please rephrase.


text:
CL604/650 (or CRJ1/2) max loiter time (using it as a reference point to compare the A-10 to)
In some discussions I was having with a (back-office) USAF acquaintance about the F-35 vs. A-10 debate, the topic of loiter time came up.  Wikipedia provides a 1.88 hour (112min) loiter time for the A-10 at a 250nmi combat radius with what appears to be a CAS loadout and a 5000' loiter altitude.  However, I'm not sure how heavily that is influenced by altitude (obviously, being at 5000' doesn't help any jet's gas mileage) vs. how much of that is being influenced by being a very "dirty" airframe laden with draggy protuberances and external stores.
So, given that the CL601/604/650 and CRJ100/200 have a similar MTOW and basically identical engines to the A-10 on a relatively "clean" airframe, assuming you had some reason to send a laden CRJ boring circles over some spot 250 nmi away at 5000' for a while (maybe it's carrying some sort of scientific payload internally), and were limited to 11klbs of fuel in doing so, how long would that CRJ be able to loiter before the flight crew had to start turning back home?

Does the A-10 switch an engine off when loitering? Not so uncommon with dual engine aircraft.

Once you're in the loiter area, the cleanliness isn't that much an issue because you're just talking about time in the air at min fuel flow, and whether that's achieved at 220 kt or 240 kt doesn't make much difference except on the trip to and from the loiter area. I dimly recall the 3B1 uses about 1100-1300 pph and maybe can be coaxed down to about 1000 pph if you slow to min drag, like .6+M.  But that's in the FLs. Down at 5000 ft it's gonna be 13-1500 pph (wild guess).  So 11k lbs might give you maybe 4+ hrs, so with 2 hrs transit and 1 hr reserve would give an hour, maybe 2, on station.


text:
What prevents fighters from being fitted with jetliner-style bleed-air wing anti-ice?
In a comment, John K points out that:

In any case, military fighter aircraft have to avoid icing, which is kind of odd for those that are considered "all-weather" interceptors..

This, to me, raises the question of "why would you omit wing anti-ice from a supposedly 'all-weather' aircraft?"  Especially on a larger fighter like a F-15 or Su-27, it seems to me that the weight penalty for a couple of piccolo tubes, two butterfly valves, and a bit of extra bleed ductwork would be fairly minimal relative to the weight of the aircraft and payload, or am I wrong on that?  Or are there aerodynamic or engine-performance reasons that prohibit fighter jets from using bleed-air anti-icing, or some other anti-ice system for that matter?  I've also heard the saw that "fighters don't need anti-ice because they can simply fly fast enough to use ram rise to deal with icing", but could there be other reasons they couldn't fly in icing, such as not having heated pitot/static systems or inlet anti-ice functions?

As you point out it would be easy enough, but fighters are designed to meet certain military specs.  If the military doesn’t require anti-ice then building it in to the design simply adds cost and puts your bid at a competitive disadvantage.  It might be a better question if you restated to ask why the military doesn’t require anti-ice.

@Michael Hall But you would think that a fighter like an F-15 designed to do interceptions in all weather would need the capability to operate in know icing conditions, at least to be able to land in icing conditions.  The only purpose built all-weather interceptor I've seen with the capability to operate in icing is the Canadian Avro CF-100, which had boots on the wings and tail.

@John K - yeah, you might think that, but fighters can get through icing fast enough that there isn’t much time for it to build.  Both climbs and descents...  HI TACAN penetrations pegged the VSI down, not something you want to do with pax.

And then there's the extra weight and complexity of the equipment that needs to be maintained and can get battle damage, and the volume taken up by which takes away from the already small volume available for mission critical things like weapons, sensors, and fuel.

Several answers to this highly related question do delve into the "why?" aspect and thus address your question as well -- https://aviation.stackexchange.com/q/58982/34686

Arguably not exactly a duplicate but practically speaking, perhaps the question has already been answered?

@MichaelHall so what about a fighter on a takeoff or arrival where speed isn't going to help?

@quietflyer the answers don't address the logical disconnect, that is, why don't "all weather interceptors" like the F-15 (as opposed to "day fighters" like the F-16) at least have the ability to operate in icing, at least through arrivals and departures.  If your opponent incorporates that capability into their attack aircraft, that's quite a limitation for an all weather interceptor.

@John K, you’ll notice that I’m not answering, because I don’t have an answer!  But apparently the pentagon brass did a cost benefit analysis and decided it wasn’t critical enough to spec...

Timely perhaps, but today we did our annual cold wx ops training for the Caravan.  We are certified for flight into known icing, yet there are still conditions we are not allowed to fly in.  Similarly, in the Navy EA6B I flew in icing many times without any anti or de-ice equipment.  Lack of equipment never stopped us from being all weather, we were just very aware and didn’t linger for long.

Well even airliners with heated LEs have to leave icing if it's heavy enough. In the CRJs  you're supposed to get out if ice is adhering to the cockpit side windows.  What intrigues me is departures and approaches in icing where you are operating at low speed.  I expect the answer is that they are considered capable of handling moderate ice loads at low speeds for approach and landing, same as lots of horizontal tails aren't anti-iced on jets because the surface is made a little bigger.  During CRJ700 development tail anti-ice was considered to allow a smaller surface, but it was rejected.

Most fighters have enough thrust to power through icing conditions, so anti-icing or heat is limited to inlet(s) and pitot probe(s). That said, at least one F-16 flight manual, to use one example, says the following about flying in known icing conditions:

Flight in areas of icing should be avoided whenever possible. If icing
conditions are anticipated or cannot be avoided, turn ANTI ICE switch
to ON and PROBE HEAT switch to PROBE HEAT. Frequently check the
aircraft leading edges for indication of ice buildup. Make all
throttle movements slower than normal when in potential icing
conditions to reduce possibility of engine stalls and/or stagnation.
Consider diverting to an alternate field if required to avoid icing
conditions.

The F-14 also had the same systems, engine inlets and pitot probe heat. No other anti-icing systems on the aircraft.